The present invention relates to ceramic shell molds for the casting of reactive metals, such as titanium, and process of making molds. In more detail, the invention pertains to a slurry containing yttria refractory used as casting compositions to form shell molds, and processes for preparing same. More particularly, the invention pertains to a slurry comprising yttria refractory, an acid, and an organic solvent, for making ceramic shell molds for casting of reactive metals such as titanium and titanium alloys. The compositions of the present invention are characterized by having a desirable shelf life and improved stability. Molds produced in accordance with the present invention are particularly useful because they enable the casting of reactive metals with minimized or essentially no alpha case.
In a further aspect, the present invention relates to ceramic and foundry cores and the yttria slurries used in making them.
Much effort has been devoted for the past 25 years to providing compositions and methods for casting reactive metals, particularly titanium and its alloys, into ceramic molds. This development and the interest in providing this capability was stimulated by activity in the nuclear and aircraft industries where it was necessary to search for high strength and lightweight metals. Titanium, because of its high strength to weight ratio is sought after for use in the aircraft industry.
The melting point of titanium metal is almost 3100.degree. F. and, in the molten condition, reacts with most refractories. Earlier attempts to cast titanium into ordinary foundry molds were unsuccessful due to the undesirable chemical reactions between the hot metal and the surfaces with which it came into contact. For example, reduction of the silica present in such foundry molds produced heavy reaction zones on the casting surface. This reaction layer is known in the industry as "alpha case" and the problems associated therewith have been described in detail in the literature.
Machined and formed graphite molds have been used commercially to make titanium castings. Such molds can be made in such a way as to minimize the alpha case layer. The U.S. Bureau of Mines has supported research for almost 25 years on the casting of refractory metals. Thus, there have been continuing efforts to search for new materials and methods to reduce or eliminate alpha case.
The use of graphite in investment molds has been described in the art in such patents as U.S. Pat. Nos. 3,241,200; 3,243,733; 3,256,574; 3,266,106; 3,296,666 and 3,321,005 all to Lirones. Other prior art which show a carbonaceous mold surface utilizing graphite powders and finely divided inorganic powders called "stuccos" are Operhall, U.S. Pat. No. 3,257,692; Zusman et al., U.S. Pat. No. 3,485,288 and Morozov et al., U.S. Pat. No. 3,389,743. These documents describe various ways of obtaining a carbonaceous mold surface by incorporating graphite powders and stuccos, various organic and inorganic binder systems such as colloidal silica, colloidal graphite, synthetic resin which are intended to reduce to carbon during burnout, and carbon coated refractory mold surfaces. These systems were observed to have the disadvantage of the necessity for eliminating oxygen during burnout, a limitation on the mold temperature and a titanium carbon reaction zone formed on the casting surface.
Further developments including variations in foundry molds are shown in Turner et al., U.S. Pat. No. 3,802,902 which uses sodium silicate bonded graphite and/or olivine which was then coated with a relatively non-reactive coating such as alumina. However, this system still did not produce a casting surface free of contamination.
Schneider, U.S. Pat. No. 3,815,658 shows molds which are less reactive to steels and steel alloys containing high chromium, titanium and aluminum contents in which a mangnesium oxide-forsterite composition is used as the mold surface.
A number of attempts have been made in the past to coat the graphite and the ceramic molds with materials which would not react with the reactive metals being cast. For example, metallic powders such as tantalum, molybdenum, columbium, tungsten, and also thorium oxide had been used as non-reactive mold surfaces with some type of oxide bond. See Brown, U.S. Pat. Nos. 3,422,880; 3,537,949 and 3,994,346.
Operhall, U.S. Pat. No. 2,806,271 shows coating a pattern material with a continuous layer of the metal to be cast, backed up with a high heat conductivity metal layer and investing in mold material.
Basche, U.S. Pat. No. 4,135,030 shows impregnation of a standard ceramic shell mold with a tungsten compound and firing in a reducing atmosphere such as hydrogen to convert the tungsten compound to metallic tungsten or tungsten oxides. These molds are said to be less reactive to molten titanium but they still have the oxide problems associated with them.
Brown, U.S. Pat. No. 4,057,433 discloses the use of fluorides and oxyfluorides of the metals of Group IIIa and the lanthanide and actinide series of Group IIIb of the Periodic Chart as constituents of the mold surface to minimize reaction with molten titanium. This reference also shows incorporation of metal particles of one or more refractory metal powders as a heat sink material. However, even those procedures have resulted in some alpha case problems.
A development by General Electric has provided barrier layers of refractory oxide in a silica bonded mold for casting alloys containing significant amounts of reactive metals; see Gigliotti et al. U.S. Pat. Nos. 3,955,616; 3,972,367 and 4,031,945.
Huseby, U.S. Pat. No. 4,240,828 shows doping a nickel and cobalt alloy with a rare earth metal and casting into a ceramic mold.
In the 1960's, developments at Wright Air Development Center led to the formation of a crucible for melting titanium formed from a titanium enriched zirconium oxide crucible with less reaction to molten titanium than pure zirconium oxide.
Richerson, U.S. Pat. No. 4,040,845 shows a ceramic composition for crucibles and molds containing a major amount of yttrium oxide and a minor amount of a heavy rare earth mixed oxide. Such methods including the making of a titanium metal enriched yttrium oxide were only partially successful because of the elaborate and expensive technique which required repetitive steps.
Molds for casting molybdenum made from zirconium acetate bonded calcia stabilized zirconium oxide have been made by the Bureau of Mines.
Feagin, U.S. Pat. No. 4,415,673 discloses a zirconia binder which is an aqueous acidic zirconia sol used as a binder for an active refractory including stabilized zirconia oxide thereby causing reaction and gelation of the sols. Solid molds were made for casting depleted uranium. A distinction is made in this patent between "active" refractories and refractories which are relatively inert. The compositions of Feagin are intended to contain at least a portion of active refractories. See also Feagin, U.S. Pat. No. 4,504,591.
Adhesive plasters made of a suspension of oxide powder, such as yttrium oxide and an acid are shown in Holcombe et al., U.S. Pat. No. 4,087,573. These compositions are described as being spontaneously hardening and useful for coating surfaces or for casting into a shape. Of particular interest is the coating of graphite crucible used in uranium melting operations.
It is generally recognized in the industry that all commercial processes have some alpha case on their casting. This may range from about 0.005 inches to 0.04 inches in thickness depending on process and casting size. The alpha case must be milled off by chemical means or other means from the casting before a satisfactory casting is obtained. The extra cost imposed by the chemical milling operation is a disadvantage and presents a serious problem from the standpoint of accuracy of dimensions. Normally, the tooling must take into consideration the chemical milling which results in the removal of some of the material in order to produce a casting that is dimensionally correct. However, since casting conditions vary, the alpha case will vary along the surface of the casting. This means that there is a considerable problem with regard to dimensional variation.
Some refractory compositions have been developed that exhibit reduced alpha case and can be used successfully to make production castings by applying the coatings to the wax patterns by special techniques, such as spraying. However, a difficulty arises in that certain refractory mixes do not have a long pot life and gel quickly, even spontaneously with stirring in a few minutes, depending upon exact composition. See Holcombe et al., U.S. Pat. No. 4,087,573.
Accordingly, it would be highly advantageous to have a slurry that is stable at least for several days and preferably for several weeks in order that patterns of the desired shape may be dipped into the slurry according to present production practice.